Fecal bacteria from 33 infants (aged 1 to 6 months) were tested for growth on commercial prebiotics. The children were born vaginally (20) or by caesarean section (13). Bifidobacteria, lactobacilli, gram-negative bacteria, Escherichia coli, and total anaerobes in fecal samples were enumerated by selective agars and fluorescence in situ hybridization. The total fecal bacteria were inoculated into cultivation media containing 2 % Vivinal® (galactooligosaccharides-GOS) or Raftilose® P95 (fructooligosaccharides-FOS) as a single carbon source and bacteria were enumerated again after 24 h of anaerobic cultivation. Bifidobacteria dominated, reaching counts of 9-10 log colony-forming units (CFU)/g in 17 children born vaginally and in seven children delivered by caesarean section. In these infants, lactobacilli were more frequently detected and a lower number of E. coli and gram-negative bacteria were determined compared to bifidobacteria-negative infants. Clostridia dominated in children without bifidobacteria, reaching counts from 7 to 9 log CFU/g. Both prebiotics supported all groups of bacteria tested. In children with naturally high counts of bifidobacteria, bifidobacteria dominated also after cultivation on prebiotics, reaching counts from 8.23 to 8.77 log CFU/mL. In bifidobacteria-negative samples, clostridia were supported by prebiotics, reaching counts from 7.17 to 7.69 log CFU/mL. There were no significant differences between bacterial growth on Vivinal® and Raftilose® P95 and counts determined by cultivation and FISH. Prebiotics should selectively stimulate the growth of desirable bacteria such as bifidobacteria and lactobacilli. However, our results showed that commercially available FOS and GOS may stimulate also other fecal bacteria.